The present invention relates to doped, pyrogenically prepared oxides, a process for their preparation and their use.
It is known that pyrogenically prepared oxides can be coated with metal salts or metal oxides by mixing the pyrogenically prepared oxides with aqueous solutions of metal salts and then drying and/or calcining.
Products prepared in this way have the disadvantages a) that the doping substance is not homogeneously distributed in the entire primary particle or b) that, depending on the type of doping, inhomogeneities may occur during mixing. Thus, after doping and calcining, the primary particles of the doping substance may separate out and be present with much larger diameters than the primary particles of pyrogenic oxides.
It is therefore an object of the invention to achieve homogeneous doping of pyrogenically prepared oxides with another substance while at the same time avoiding problems of the prior art, and more particularly, to avoid the presence of separate primary particles of the doping substance or oxides of the doping substance alongside primary particles of the pyrogenically prepared oxide.
The above as well as other objects are obtained by the present invention in the form of doped, pyrogenically prepared oxides of metals and/or non-metals wherein the basic components are pyrogenically prepared oxides of metals and/or non-metals, prepared using flame hydrolysis techniques, which are doped with at least one doping component at 0.00001 to 20 wt. %, wherein the doping amount may preferably be in the range 1 to 10,000 ppm, and the doping component is a non-metal and/or a metal or a non-metal salt and/or a metal salt or an oxide of a metal and/or a non-metal, and the BET surface area of the doped oxides is between 5 and 600 m2/g.
Another feature of the invention is a process for preparing doped, pyrogenically prepared oxides of metals and/or non-metals. In carrying out the process, an aerosol is fed into a flame, such as is used in a known manner to prepare pyrogenic oxides by flame hydrolysis and wherein this aerosol is homogeneously mixed with the gas mixture for flame oxidation or flame hydrolysis prior to reaction. The aerosol/gas mixture is allowed to react in the flame and the resulting doped pyrogenically prepared oxides are separated from the gas stream in a known manner. A salt solution or suspension which contains the components of the substance to be doped, which may be a metal salt or a non-metal salt (metalloid salt) or mixtures of both or a suspension of an insoluble metal compound or non-metal (metalloid) compound, is used as the starting material for the aerosol. The aerosol is prepared by nebulization using a two-fluid nozzle or using an aerosol generator, preferably by the ultrasonic method.